package/wavemon: add libcap optional dependency

libcap is an optional dependency which is enabled by default since
version 0.7.1 and
https://github.com/uoaerg/wavemon/commit/669c53e3359af5f6d4044126e22ab95f40a968f4
and can be explicitly enabled or disabled since version 0.9.0 and
https://github.com/uoaerg/wavemon/commit/af36fbe75649f745e205cd961302b656b8461e06



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>